1. Technical Field
The present invention relates to an array substrate and a liquid crystal display device. More specifically, the present invention relates to an array substrate in a fringe field switching (hereinafter also referred to as FFS) type and a configuration of a liquid crystal display device using the same.
2. Background
Recently, instead of conventional cathode-ray tubes, new display apparatuses having a thin and flat display panel using such as liquid crystal, electroluminescence and charged particles, etc., is widely used. A liquid crystal display device, which is representative of the new display apparatuses, has not only a thin and lightweight but also low power consumption and low voltage driving characteristics. The liquid crystal display device has two substrates and the liquid crystal is sealed therebetween. One substrate is an array substrate having a display area in which a plurality of pixels is arranged in a matrix shape, and the other substrate is a counter substrate having a color filter, a black matrix (shielding plate), etc., formed thereto.
Superficially, in a thin film transistor (hereinafter also referred to as TFT) type liquid crystal apparatus, the TFT, which is a switching device, is provided in the respective pixels on the array substrate to maintain a voltage, which independently drives the liquid crystal of the respective pixels. Therefore, it is possible to display high-definition image having little crosstalk. Further, the respective pixels are provided with a scanning line (gate line), which controls of the TFTs state between ON and OFF, and a signal line (source line) intersecting with the scanning line to input image data. Generally, the respective pixels correspond to respective areas surrounded by the scanning line and the signal line.
In an in-plane switching (hereinafter referred to as IPS) liquid crystal display device, a plurality of pixel electrode and a plurality of counter electrode (common electrode) are alternately arranged at an interval on one array substrate, and an electric field is applied in a direction substantially horizontal to the substrate to display the image. The IPS type has a superior view angle and has an advantage over a typical twisted nematic (TN) type. However, the related IPS liquid crystal display device has a disadvantage that light transmittance is inferior to the typical TN type.
In order to solve the above problem, a fringe field switching (FFS) type has been considered (for example, see JP-A-11-202356, FIGS. 19 to 21). In the FFS type liquid crystal display device, a fringe electric field (oblique electric field including both components of a horizontal electric field and a vertical electric field) is applied to the liquid crystal to display the image. In the FFS liquid crystal display device, the pixel electrode and the counter electrode are formed on one array substrate, similar to the IPS type. However, the pixel electrode and the counter electrode are vertically arranged, and an insulating film is interposed therebetween. Typically, the lower electrode has a plate shape, and the upper electrode has a slit shape or a comb shape that have a gap part and a branch electrode part.
In the FFS type LCD, either the lower electrode or the upper electrode can be configured as the pixel electrode. Since the liquid crystal is driven by the fringe electric field between the upper electrode and the lower electrode in the FFS type LCD, the liquid crystal on the branch electrode part of the upper electrode can be also driven and be served to display. Thus, the light transmittance is improved in the FFS type LCD, compared to the IPS type LCD in which a part of the pixel electrode and the counter electrode scarcely serve to display.